Because of having a high reflectivity (88% to 90%), a pure Al film is used as a reflective film for a vehicular lighting fixture, a lighting device, an ornament, or the like. Durability required of a reflective film includes mild acidity resistance, alkali resistance, warm water resistance, heat resistance, moisture resistance, sulfidation resistance, salt water resistance, and the like. However, since the pure Al film is made of an amphoteric metal, the corrosion resistance thereof to an acid or alkali is low. As a result, when a reflective film (pure Al reflective film) formed of a pure Al film is used for a vehicular lighting fixture or the like, a problem arises that the reflective film deteriorates in a short period or time, and cannot retain a high reflectivity over a long period of time.
As a method for maintaining the high reflectivity of the pure Al reflective film for a long period of time, there is a method which forms a protective film having corrosion resistance to an acid or alkali on a surface of a pure Al reflective film. However, in such a method, if a defect such as a pinhole exists in the protective film, corrosion (oxidation or elution) of the pure Al film may occur via the defective portion to cause a reduction in reflectivity. The reduction in reflectivity resulting from a defect such as a pinhole can be inhibited by increasing the thickness of the protective film, but the productivity of a product lowers to increase cost. Therefore, the thickening of the protective film is not an effective means.
In view of the circumstances, Japanese Unexamined Patent Application Publication No. Hei 11 (1999)-221517 discloses a method which forms a protective film (top-coat) in which a defect such as a pinhole is less likely to occur on a surface of a reflective film. Specifically, a method is disclosed in which a monomer of a top-coat material such as HMDS (hexamethyldisiloxane) or TEOS (tetraethylorthosilicate) is introduced into a vacuum vessel under a pressure environment under which a plasma is highly likely to be generated, and the monomer mentioned above is polymerized in the plasma to be precipitated on the surface of the reflective film to provide the top-coat on the surface of the reflective film. However, in such a method, the maintenance (such as cleaning of the inside of the vacuum vessel) of an apparatus used to form the protective film (top-coat) is required, and consequently the productivity of a product may lower. In addition, Japanese Unexamined Patent Application Publication No. Hei 11 (1999)-221517 described above also discloses a laminate in which an under-coat, a plasma processed layer, and a top-coat are successively formed over a resin base material, which leads to the problem of a reflectivity reduction due to an increased thickness.
Besides, as a method for maintaining the high reflectivity of the pure Al reflective film for a long period of time, a method which alloys Al with another element is also disclosed. For example, Japanese Unexamined Patent Application Publication No. Hei 07 (1995)-301705 discloses an Al alloy reflective film doped with a IIIa-group, IVa-group, Va-group, VIa-group, VIIa-group, or VIII-group transition metal element in the periodic table. The Al alloy reflective film shows high corrosion resistance in an acidic to neutral range but, according to the alloy design concept therefor, a chemically stable passive coating is formed so that no consideration has been given to an improvement in corrosion resistance in an alkaline range in which the dissolution of the passive coating advances corrosion. As a result, a problem arises that, if there is a defect such as a pinhole, corrosion (elution due to an alkali) of the Al alloy reflective film occurs via the defective portion to reduce the reflectivity.
In the meantime, a vehicular lighting fixture such as a headlamp or a rear lamp has the problem that dew drops and water droplets are highly likely to be formed therein. This is because outside air containing moisture flows into the vehicular lighting fixture, and the vehicular lighting fixture is heated by heat generated from a light source during a lighting period, while being cooled by the outside air or rain during a non-lighting period, thereby being subjected to repeated heating and cooling. The formation of the water droplets causes the reflective film provided in the vehicular lighting fixture to become transparent (oxidized), and causes a reduction in the reflectivity thereof or the like. Therefore, there is strong demand for a reflective film which is excellent not only in mild acidity resistance and alkali resistance, but also in warm water resistance. Similar problems are also observed in a lighting device, an optical mirror, and the like.